Stents are used in a variety of medical procedures. For example, stents are often used in connection with assisting drainage from the kidney through the ureter, from the liver through the biliary ducts, from the gall bladder through the cystic, hepatic, or common bile ducts, dorsal or ventral pancreas through the pancreatic ducts, and the like. A leading reason for stent deployment in ducts is to provide drainage to circumvent a blockage. Blockage of ducts in the body can be a serious and very painful affliction that can result in death if not promptly and effectively treated. Blockages can occur for a number of reasons. For example, in the kidney and gall bladder, stones, or debris from such stones, can pass into the ureter or the bile ducts where they become entrapped. Alternatively, cysts or tumors growing against the outer wall of the ducts can cause constriction of the ducts. Similarly, internal or duct wall cysts or tumors can act to block ducts.
In many cases, the problem is solved by surgery, medication, or waiting until debris is naturally cleared from the duct. However, a stent must often be inserted in the duct on at least a temporary basis to provide drainage until the condition can be corrected.
Similarly, blood vessel stents are often used in grafting and supporting blood vessel tissues following invasive medical procedures, such as vascular surgery and angioplasty. Similar concerns are also raised in the catheter and intubation arts, in general, which include, without limitation: intravenous catheters, guiding catheters, sheaths, umbilical catheters, trocar catheters, heart catheters including, valvostomy catheters, angioplasty catheters, arthroscopy catheters, and the like), perfusion catheters, suction catheters, oxygen catheters, endoscopy catheters, endotracheal tubes, stomach tubes, feeding tubes, lavage tubes, rectal tubes, urological tubes, irrigation tubes, aneurysm shunts, stenosis dialators, trocars, and inserters.
Looking in particular at ureteral stents by way of example, there are many different stents available. The main function of each of these ureteral stents is to bypass ureteral obstruction and to provide urary drainage from the kidney to the bladder for a period of time which varies but is usually of the order of a few days to several months.
There are several methods of stent placement within the ureter. One method involves passing a guide wire up the ureter into the kidney. Thereafter, a tubular stent is fed and coaxially slid up the guide wire into the ureter using a tubular stent pusher. An alternate method employs placing a tubular stent having a closed or partially tapered shut proximal end over a guide wire. The stent is thereafter advanced up into the ureter by pushing the guide wire against the closed or partially tapered shut end. Another alternate method is to place the tubular stent over the guide wire with the stent pusher over and affixed to the guide wire behind the stent and thereafter to advance the entire assemblage into the ureter. These methods can also be used, with appropriate surgery to provide access, to insert a stent from the kidney downwardly through the ureter to the bladder.
Early ureteral stents were straight. As a result, after placement into the ureter, these straight stents often migrated or were expelled from the ureter as a result of peristaltic action by the ureter. Later ureteral stents, therefore, were usually designed with means of retention on one or both ends of the stent. The retention means is intended to inhibit stent migration either upward into the kidney or downward into the bladder. Retention means that have been employed are in the form of hooks, pigtails, coils, corkscrews, malecots, barbs, mushrooms, or any other practical shape that will serve the purpose.
Ureteral stents also come in many different lengths. The variations in stent length are often necessary to accommodate the different ureter lengths in different size patients. As a result, a stock of different length ureteral stents must often be kept available. To overcome this problem of stocking many different length ureteral stents, some stents have been designed in the form of an expanding coil or corkscrew as disclosed in U.S. Pat. Nos. 4,531,933; 4,643,716; 4,671,795; and 4,813,925, or utilize connectors as disclosed in U.S. Pat. No. 4,790,810.
In addition to varying lengths, ureteral stents are also made with varying diameters, e.g., from 3 French (1 mm) to 16 French (5.28 mm), and typically, 4.5 French (1.5 mm) to 8.5 French (2.8 mm), and varying degrees of hardness. Ureteral stents with smaller diameters are usually easier to insert but may not provide sufficient drainage, whereas stents with larger diameters allow for increasing drainage capacity through the ureter but may be difficult to insert. Stiff ureteral stents are also easier to insert than are softer stents, but once inserted can lead to increased patient discomfort. Softer stents, on the other hand, provide more comfort for the patient but are more difficult to insert due to their softness. Presently, most available stents are either made of silicone as disclosed in U.S. Pat. No. 4,212,304 or of a harder polymer. Silicone may increase patient comfort, but because of the softness of silicone, it is more difficult to guide the stent into the ureter. Once in the ureter, the softness of the silicone increases the likelihood of migration of the stent because rigid retention means are not available.
To balance ease of insertion, better retention and patient comfort, some ureteral stents have been designed combining a stiff material at the kidney end for easier insertion and better retention with a softer material at the bladder end for patient comfort. These dual hardness stents are disclosed in U.S. Pat. Nos. 4,820,262; 4,874,360; and 4,931,037.
It is at times desirable or necessary to provide a stent which is wider at one end, either its proximal end or its distal end, perhaps as much as 16 French in diameter, and narrower at the other end, perhaps 4.5 French to 7 French. In the past, this has usually required insertion from the proximal (kidney) end of the ureter, a relatively difficult procedure.
Swellable ureteral stents utilizing hydrophilic polymers of the nature set forth in U.S. Pat. No. 4,377,010 and elsewhere, generally as coatings on other materials but also alone, have been investigated using piglets (See An Experimental Study of Hydrophilic Plastics for Urological Use, J. W. A. Ramsey, et al, British Journal of Urology, Volume 58, pp 70-74, 1986 and/or Evaluation of Polymeric Materials for Endourologic Devices, H. K. Mardis, Seminars in Interventional Radiology, Volume 4. Number 1, pp 36-45, March 1987) but have not received acceptance in the medical community. Such stents have not been formulated with different softnesses and/or swellabilities at different portions thereof whereby optimal comfort combined with retainability, ease of insertion and the ability to provide stents which will assume specially desired shapes on hydrating have not been available or contemplated.
Similar problems described above in respect of ureteral stents exist in the art of stents in general. Indeed, many of the aforementioned problems are common to a variety of medical devices that are inserted or implanted in a patient.
Certain work has been done in shape memory technology. For example, certain shape memory metals exist, such as Nitinol. Shape memory has been simulated using certain hydrophilic polymers, i.e., in the context of softening and expanding materials. Mardis, supra. Recently, in U.S. Pat. No. 5,234,457 to Anderson, a type of shape memory was used in intravenous stents. There, a metallic mesh stent was compressed and impregnated with a softenible material, such as a gelatin or a resorbable polymer. The stent, upon softening of the softenible material, would expand against the artery or vein.
Thus, although stents and medical devices have been designed to address one or more of the above problems specifically, there are currently no devices incorporating features that can be used to bypass most of the aforementioned disadvantages. It would thus be desirable to have a medical device that provides one or more of the following attributes, easy insertion or implantation, selectable and different degrees of softening and/or swelling on different portions of the stent, a tapered tip that expands to an adequately large size once expanded, strong retention, insertable or implantable into a small space yet can, if desired, assume a different configuration, size, or shape (i.e., such as a significantly larger diameter at the distal and/or the proximal end upon hydration or another retention means), and, at the same time, increases patient comfort.